Preparation and characterization of clay-based porous ceramics with boric acid as additive

Intention of this work was to obtain porous silicon dioxide ceramics by using boric acid as an inexpensive additive at low forming pressure and low sintering temperature. Starting raw material, smectite clay from surface coal mine Kolubara, Serbia, was purified from organic and inorganic impurities by using heat and chemical treatment. Boric acid was used as binding and sintering aid in amount of 0.5, 1 and 2 wt%. Powder was compacted by using different pressures: 40, 60 and 80 MPa. Pressed samples were sintered at 850, 1000, 1150, and 1300 °C for 4 h in air. A relatively high porosity of nearly 40% is obtained for the samples pressed at 40, 60 and 80 MPa and sintered at 1000 °C. Median pore size diameters are in the range of macroporous up to 0.2 μm and 10 μm in the samples sintered at 1150 and 1300 °C, respectively. X-ray diffraction (XRD), scaning electron microscopy (SEM), and porosimetry measurements were employed to characterize the phases and microstructure of the obtained ceramics. The relations between mechanical characteristics of samples (Young modulus and Poisson ratio) and content of boric acid were studied.